Bull Volcanol (2014) 76:811 DOI 10.1007/s00445-014-0811-0 RESEARCH ARTICLE The history and dynamics of a welded pyroclastic dam and its failure Graham D. M. Andrews & James K. Russell & Martin L. Stewart Received: 17 July 2013 /Accepted: 15 February 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract The 2,360 BP eruption of Mount Meager, British approximately 8 h and eroded a 2.5-km long canyon into the Columbia began as an explosive, dacitic sub-Plinian eruption still-hot dam core before returning to background flow rates. that waned rapidly to a sustained period of Vulcanian, eruption-triggered dome collapse events producing volumi- Keywords Block and ash flows . Volcanic dams . Welding . nous block and ash flow (BAF) deposits. The earliest BAF Dam failure . Lahar deposits accumulated rapidly enough immediately downslope of the vent to retain heat and weld; using the deposit as a paleoviscometer determines an effective viscosity of 109– Introduction 1010 Pa s during welding. This prolific production of hot lava and block and ash flows, in a steep mountainous terrain, Pyroclastic density currents and volcanic mudflows (“lahars”) created a ∼110 m high, largely impermeable dam capped by account for 40 to 50 % of all fatalities related to volcanic permeable, non-welded BAF deposits and unconsolidated eruptions (Auker et al. 2013) including the greatest volcanic avalanche deposits that blocked the flow of the Lillooet disasters of the twentieth century (Mont Pelée, Martinique, River and created a temporary lake. The welded pyroclastic 1902; Nevada del Ruiz, Colombia, 1985). Although pyroclas- dam was compromised and overtopped at least once before tic density currents have the potential to cause tens of thou- the peak dam height was reached. Renewed eruption caused sands of fatalities as infrequent events, lahars cause tens to buildup of the dam to a maximum of 780 m above sea level hundreds of fatalities annually (Auker et al. 2013). Lahars are (asl) and grew the temporary lake to an elevation of 740 m asl particularly challenging hazards to plan for, and mitigate and a minimum volume of 0.55 km3. The rise of lake level led against, because they can occur during and for several years to catastrophic failure of the top of the dam, generating an after an eruption (Manville and Cronin 2007). Many lahars outburst flood that carved a canyon through most of the dam result from the post-eruption failure of temporary lava- and resulted in a voluminous lahar that is traced at least 65 km generated dams (e.g., Fenton et al. 2006) and volcaniclastic- downstream. Based on current flow rates of the Lillooet River, generated dams (Capra 2007), and the associated catastrophic the lake would have overtopped the final dam at a minimum of release of the impounded lake. Examples include the 1982 39–65 days after its formation. The peak deluge lasted eruption of El Chichón, Mexico (Macías et al. 2004); Nevado de Colima, Mexico (Capra and Macías 2002); Ruapehu 2007, New Zealand (Manville and Cronin 2007); and Numazawa, Editorial responsibility: V. Manville Japan (Kataoka et al. 2008). In this paper, we describe and interpret the proximal and * G. D. M. Andrews ( ) medial volcanic deposits associated with damming of the Department of Geosciences, California State University Bakersfield, Bakersfield, CA 93311, USA Lillooet River during the 2,360 BP eruption of Mount e-mail: [email protected] Meager, B.C. We use field observations to reconstruct the : : temporal and spatial building of the dam, including the height G. D. M. Andrews J. K. Russell M. L. Stewart and duration of the dam, the size of the lake, and the down- Volcanology & Petrology Laboratory, Department of Earth & Ocean Sciences, University of British Columbia, Vancouver, BCV6T 1Z4, stream extent of the outburst debris flow deposits. Our work Canada provides semiquantitative estimates on the rates, volumes, and 811, Page 2 of 16 Bull Volcanol (2014) 76:811 timescales of dam-building, lake-filling, and dam failure. This Fig. 2 a Map of the 2,360 BP Pebble Creek Formation (modified from is the first described occurrence of a partially welded pyro- Hickson et al. 1999). b Non-exaggerated oblique south-southwest view of clastic dam and a rare case where field evidence can elucidate the Mount Meager volcanic complex (MMVC) and Lillooet River valley (adapted from Google EarthTM). c Panorama and dimensions of the the timescales of events preceding and attending the failure of canyon incised into unit A; viewed downstream (east-southeast) from a volcanic dam (cf. Capra 2007). Keyhole Falls Geological setting The southern Coast Mountains of British Columbia (Fig. 1) have been characterized by rapid uplift rates and enhanced Mount Meager volcanic complex glaciofluvial erosion rates over the past 4 Ma (Clague et al. 1982;Andrewsetal.2012). The MMVC is now highly The 2,360 BP eruption of the Mount Meager volcanic com- dissected and its base is perched at 1,100–1,200 m above plex (MMVC), Cascades arc, British Columbia (Fig. 1; Read sea level (asl), more than 500 m above the present-day erosion 1977) is the youngest explosive volcanic event in Canada surface marked by the base of the adjacent Lillooet River (Hickson et al. 1999). The eruptive center poses a significant valley (400–600 m asl; Fig. 2). The 2,360 BP eruption vent risk to local communities from both volcanic eruptions and is situated immediately above the narrow and steep-sided, non-volcanic mass-wasting events (Simpson et al. 2006; glaciated valley hosting the Lillooet River, and, as a conse- Friele et al. 2008). quence, this portion of the Lillooet River valley is partially filled with a thick (0–180 m) accumulation of proximal pyro- clastic deposits. MT. MEAGER Physiography of Upper Lillooet River valley The Upper Lillooet River valley (Fig. 2a) is a steep-sided, narrow, glaciated valley situated immediately north of the British ∼ Columbia MMVC and extending 16 km upstream towards the Lillooet icefield (Fig. 2b). The Lillooet drainage was sculpted by Late EXPLORER inset Pleistocene glaciation; however, the post-glacial physiography PLATE of the Lillooet River is significantly different along a ∼2.5 km stretch immediately below the MMVC and southeast of Keyhole Falls (Fig. 2a), where it has been infilled by deposits Washington of the Pebble Creek Formation (PCF: Hickson et al. 1999). At Keyhole Falls, two morphologically distinct canyons JUAN DE FUCA are carved into PCF deposits—(1) a <150 m deep, ∼300 m PLATE wide, horseshoe-shaped canyon (Fig. 2c) carved through the entire thickness of the PCF to the basement and (2) a 60 m deep, <15 m wide slot canyon carved into welded BAF MOUNT MEAGER Oregon VOLCANIC deposits (Fig. 3a). The larger horseshoe-shaped canyon is COMPLEX straight, rough sided, rubble strewn, and extends approximate- Lillooet ly 2.5 km downstream of Keyhole Falls. The smaller slot River canyon is short (<1 km in length), sinuous, and has water- Pemberton polished vertical surfaces and extends upstream of Keyhole Falls. Mt. Cayley The slot canyon is typical of perennial water-carved slot Whistler canyons (e.g., Carter and Anderson 2006) resulting from California steady long-term stream flow and abrasion. However, the scale, shape, and textural morphology of the larger canyon Mt. Garibaldi (Fig. 2c) are atypical of fluvially carved canyons. The width Squamish and depth of the canyon are at least an order of magnitude 0 20 km 0 200 km larger than can be supported by the present-day fluvial dis- charge. Such misfit stream–canyon relationships commonly Fig. 1 Map of the Cascade volcanic arc (triangles) of western North America, in southern British Columbia, Canada, and location of Mount indicate rapid and sudden alteration of local drainage systems Meager (inset) within the Garibaldi volcanic belt (e.g., local tectonism, enhanced glacial erosion, or outburst Bull Volcanol (2014) 76:811 Page 3 of 16, 811 a 2200 2000 1800 1600 Salal Creek 800 Lillooet 1200 1400 River 1000 N 800 Keyhole Falls 1000 1200 1400 1600 PCF inferred vent 1000 2000 800 2200 Plinth Peak 2400 600 Meager Creek 400 b Lillooet Icefield MMVC inferred vent Lillooet River Meager Creek Lillooet River Keyhole Falls Salal Creek N c non-welded block & ash flow and sedimentary deposits 90 m welded block & ash flow deposit ~300 m 811, Page 4 of 16 Bull Volcanol (2014) 76:811 a simplified lithostratigraphy at Keyhole Falls 780 non-welded block and ash flow deposits unit C BAF, gravel, and rock avalanche 700 slot-canyon deposits unit B resistant 3C surface welded block 3B and ash flow deposits unit A 600 m asl b c unit C welded BAF deposit non-welded BAF unit B & rock avalanche deposits welded BAF deposit sand and gravel unit A Fig. 3 a The pyroclastic dam at Keyhole Falls (looking northwest) and volcaniclastic succession (unit B) buried by a later non-welded BAF the Keyhole Falls slot canyon. The locations of photos b and c are shown. deposit (unit C); note the presence of alternating recessive (soft, non- b Close-up of irregularly fanning columnar joints in unit A in the canyon welded BAF, sand, gravel, and rock avalanche deposits) and cliff-forming wall at Keyhole Falls (courtesy of Paul Adams); the slot canyon is 60 m (hard, welded BAF deposits) horizons deep. c View of the northern canyon wall and the 45-m thick clastic and floods). Other examples in volcanic settings include down- et al. 1996; Hickson et al. 1999). The intensity of the eruption stream of Lake Taupo (Manville et al. 1999;Manvilleand waned to a Vulcanian explosive phase (Michol et al. 2008) Wilson 2004) and Lake Tarawera in New Zealand (Hodgson producing a sequence of welded (early) and non-welded (late) and Nairn 2005;Manvilleetal.2007).